Glow discharge deposition is employed for the preparation of thin films of a variety of materials such as semiconductors, insulators, optical coatings, polymers and the like. In a typical glow discharge deposition, a process gas, which includes a precursor of the material being deposited, is introduced into a deposition chamber, typically at subatmospheric pressure. Electromagnetic energy, either AC or DC, is introduced into the chamber and energizes process gas so as to create an excited plasma therefrom. In the plasma, the precursor material is decomposed to create deposition species which deposit a coating on a substrate maintained proximate the plasma region. Frequently, the substrate is heated to facilitate growth of the deposit thereupon. This technology is well known in the art. Early glow discharge deposition processes employed either direct current, a low frequency alternating current or a radio frequency alternating current to energize the plasma. Radio frequency current is still very widely employed for this purpose.
One particular drawback to radio frequency energized glow discharge deposition processes is their relatively low rate of disposition and inefficient utilization of process gas. In an attempt to increase deposition rates and gas utilization, those of skill in the art have turned to the use of microwave energized plasmas. It was found that while microwave energized processes greatly improve deposition rates and gas utilization, materials, particularly semiconductor materials, deposited therefrom were generally of poorer quality than corresponding radio frequency deposited materials. Conventional wisdom heretofore has been that lowered material quality is inherent in microwave energized depositions.
Microwave processes, because of their efficiencies, are quite attractive for the high volume, commercial production of semiconductor devices, particularly photovoltaic devices, and various approaches have been implemented to accommodate the heretofore reduced quality of microwave deposited semiconductor layers. In some instances, multilayered semiconductor devices are manufactured utilizing a mixture of microwave deposited and radio frequency deposited layers, and by an appropriate combination of layers, the effect of reduced material quality is minimized. This approach represents a trade off of device efficiency and the economics of manufacturing; furthermore, the use of both radio frequency and microwave deposited layers in a single device, complicates both the production process and the equipment for its implementation. It would clearly be desirable to improve the efficiency of microwave deposited layers and to avoid the use of mixed microwave/radio frequency depositions in a production mode. Furthermore, it is always desirable to improve the quality of the semiconductor material in devices. In photovoltaic devices, material quality correlates directly with device efficiency; therefore it would be most desirable to have a microwave energized deposition process which produces photovoltaic semiconductor material of a quality which exceeds that of materials deposited by conventional radio frequency processes.
Since the prior art regards microwave deposited semiconductor material as being of lesser quality than radio frequency deposited material, it does not show any methodology whereby a microwave process may be employed for depositing semiconductors having material qualities equal to, or exceeding those of radio frequency deposited semiconductors of corresponding composition. The prior art thus teaches away from the principles of the present invention. For example, U.S. Pat. No. 4,504,518 describes various processes for the microwave energized deposition of semiconductor layers. The depositions are primarily carried out at fairly high rates and it is noted in the patent that a low deposition rate, microwave energized process provides poor quality semiconductor material. U.S. Pat. No. 4,517,223 describes another prior art, microwave energized deposition process. U.S. Pat. No. 4,721,663 describes the preparation of microcrystalline silicon by microwave deposition at a pressure of 0.1 torr at a deposition rate of 20 angstroms per second. None of the foregoing disclose or anticipate the manufacture of very high quality semiconductor material in a microwave energized deposition. The copending U.S. patent application Ser. No. 844,372, filed Feb. 24, 1992, discusses the fact that microwave deposited semiconductor alloys are of low quality compared to radio frequency deposited alloys and describes the manufacture of semiconductor devices by a combination microwave/radio frequency process wherein certain relatively thick layers of the device are deposited by a microwave process and wherein other layers, which are more critical to efficient device function, are prepared by a less efficient radio frequency process.
The present invention provides a method whereby a microwave energized deposition process may be employed to prepare semiconductor material which exceeds the quality of material prepared by a corresponding radio frequency deposition process. The method of the present invention, as will be described in greater detail hereinbelow, controls the plasma composition and deposition rate to enhance the overall quality of the semiconductor material prepared therein. The present invention may be implemented into conventional microwave energized deposition equipment and its use results in the economical manufacture of very high quality semiconductor devices. These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow.